December 10, 2007

When the mapping of the human genome was completed in 2003, researchers discovered a shocking fact: our bodies are littered with the shards of retroviruses, fragments of the chemical code from which all genetic material is made. This discovery has created a new discipline, paleovirology, which seeks to better understand the impact of modern diseases by studying the genetic history of ancient viruses.

Highly infectious viral diseases -including the Plague, yellow
fever, measles, smallpox and he Spanish Flu, which killed 50 million
people at the end of the First World War, moving from one cell to the
next, transforming each new host into a factory that makes even more
virus. In this way, one infected cell soon becomes billion -that die
when the host dies.

Endogenous retroviruses, however, once they infect the DNA of a
species they become part of that species: they reside within each of
us, carrying a record that goes back millions of years. Molecular
battles of endogenous retroviruses that raged for thousands of
generations, have been defeated by evolution.

These viral fragments are fossils that reside within each of us,
carrying a record that goes back millions of years. Because they no
longer seem to serve a purpose or cause harm, these remnants have often
been referred to as “junk DNA.” Although many of these evolutionary
relics still manage to generate proteins, scientists have never found
one that functions properly in humans or that could make us sick.

That is until Thierry Heidmann who runs the laboratory at the
Institut Gustave Roussy, on the southern edge of Paris, brought one to
life. Heidmann long suspected that if a retrovirus happens to infect a
human sperm cell or egg, which is rare, and if that embryo
survives—which is rarer still—the retrovirus could have the
evolutionary power to influence humans as a species becoming part of
the genetic blueprint, passed from mother to child, and from one
generation to the next, much like a gene for eye color or asthma.

In a brilliant essay if The New Yorker, author Michael Specter
brings Heidmann's discovery to life, showing how by "combining the
tools of genomics, virology, and evolutionary biology, he and his
colleagues took a virus that had been extinct for hundreds of thousands
of years, figured out how the broken parts were originally aligned, and
then pieced them together. After resurrecting the virus, the team
placed it in human cells and found that their creation did indeed
insert itself into the DNA of those cells. They also mixed the virus
with cells taken from hamsters and cats. It quickly infected them all,
offering the first evidence that the broken parts could once again be
made infectious. The experiment could provide vital clues about how
viruses like H.I.V. work. Inevitably, though, it also conjures images
of Frankenstein’s monster and Jurassic Park."

Heidmann named the virus Phoenix, after the mythical bird that rises
from the ashes, because he is convinced that this virus and others like
it have much to tell about the origins and the evolution of humanity.

“This is something not to fear but to celebrate,’’ Heidmann told
Specter one day as they sat in his office at the institute, which is
dedicated to the treatment and eradication of cancer.“What is
remarkable here, and unique, is the fact that endogenous retroviruses
are two things at once: genes and viruses. And those viruses helped
make us who we are today just as surely as other genes did. I am not
certain that we would have survived as a species without them. The
Phoenix virus sheds light on how H.I.V. operates, but, more than that,
on how we operate, and how we evolved. Many people study other aspects
of human evolution—how we came to walk, or the meaning of domesticated
animals. But I would argue that equally important is the role of
pathogens in shaping the way we are today. Look, for instance, at the
process of pregnancy and birth.’’

Heidmann believes that without endogenous retroviruses mammals might
never have developed a placenta, which protects the fetus and gives it
time to mature, which eventually led to live birth, one of the
hallmarks of human evolutionary success over birds, reptiles, and fish.
Eggs cannot eliminate waste or draw the maternal nutrients required to
develop the large brains that have made mammals so versatile. “These
viruses made those changes possible,’’ Heidmann told me. “It is quite
possible that, without them, human beings would still be laying eggs.”